Discrete (non-integrated) power semiconductor devices are often built using a vertical structure in which electrical current passes through the thickness of the chip between the front and back surfaces of the device. The front surface of such devices presents an anode or “front metal,” the back surface presents a cathode or “back metal,” and each electrode takes the form of a metal pad comprising several layers of metal. Alternatively, the front surface presents the cathode, and the back surface presents the anode. Because these metal layers are deposited on top of any other layers, both front metal and back metal are often referred to herein as “top metal.” Some devices present more than one electrode on one or both sides. For example, bipolar transistors (BJTs) and power MOSFETs are three-terminal devices that present two electrodes, such as an emitter and a base (in the case of BJTs) or a source and a gate (in the case of MOSFETs), on their front surfaces.
Titanium-nickel-gold and titanium-nickel-silver top metal systems are used in the silicon power semiconductor industry to mount chips using high temperature soldering. Unfortunately, both systems suffer from significant disadvantages: the former from the increasing cost of gold, and the latter from the strong tendency of silver to tarnish even when a great deal of care is taken in transport and storage. Furthermore, both systems, when used as top layers, are prone to scratching due to their low hardness (both are between 2.5 and 3 on the Mohs scale), which can result in the costly rejection of wafers. Additionally, where the electrodes are exposed to multiple iterations of the soldering cycle, gold will interact with and allow the nickel to migrate to the outer surface of the metal system where it may corrode and compromise the soldered connection.